A typical hydraulic lash adjuster (HLA) P1 as known in the art is shown in FIG. 1. The HLA P1 comprises a first oil-containing pressure chamber P2 defined between an outer body P3 and a plunger assembly P4 slidably mounted within the outer body P3, and a spring P5 arranged to enlarge the first chamber P2 by pushing the plunger assembly P4 outwardly from the outer body P3 to extend the HLA P1 to take up slack in a valve train assembly. The plunger assembly P4 defines a second oil containing pressure chamber P6 which is in fluid communication with the engine's oil supply (not shown). An aperture P7 between the first chamber P2 and the second chamber P6 allows oil to flow from the second chamber P6 into the first chamber P2, via a one way valve P8, when the HLA P1 extends. The one way valve P8 comprises a ball P8a captured by a cage P8b and biased by a spring P8c to a position closing the aperture P8. As the plunger assembly P4 moves outwardly, the volume of the first chamber P2 increases and a resulting oil pressure differential across the ball P8a moves it against the bias of the spring P8c, opening the aperture P7 and enabling oil to flow from the second oil chamber P6 into the first oil chamber P2. When the plunger assembly P4 stops moving outwardly, and the oil pressure across the ball P8a equalises, the ball P8a closes the aperture P7 under the action of the spring P8c. 
Accordingly, a typical HLA can extend to accommodate any slack in a valve train assembly, such as between a cam and a roller, but, after it is extended, the incompressible oil in the first chamber P2 provides sufficient rigid support for the HLA P1 to open a valve when, for example, a rocker arm pivots under the control of a cam (i.e. the incompressible oil prevents the plunger assembly P4 being pushed back inwardly of the outer body P3 so that the HLA P1 acts as a solid body). The oil can escape the first chamber P2 only slowly, for example, via a small annular ‘leak-down’ gap P9 defined by closely spaced leak down surfaces of the outer body P3 and the plunger assembly P4. This oil leakage down the leak down surfaces from the first chamber P2 allows the HLP P1 to retract again.
Typically, HLAs (such as P1) are ‘standalone’ devices, and are positioned between two components of the valve train. In the configuration shown in FIG. 1, HLA P1 is installed between, for example, a valve, a valve bridge that carries a pair of valves or a push rod that carries a valve illustrated schematically as P11 and a rocker arm illustrated schematically as P10 of a valve train.
In a typical arrangement, the HLA P1 is housed in an aperture of a rocker arm with the bottom of the outer body P3 extending out from that aperture.
Such an arrangement can limit the compactness of engines due to the space that the arrangement consumes. Moreover, there is typically a requirement in engine design that moving parts in an engine (for example, components such as a rocker arm containing a HLA) should not pass closer than a minimum distance (e.g. 2.5 mm) to a static part of the engine (for example, a fuel injector). In fulfilling this requirement, therefore, the space consumed by a typical HLA such as P1 (which is both a moving part and has a minimum size for a given load) can limit the overall compactness of the engine design.
There are limits on the extent to which the size of a typical HLA such as P1 can be reduced. For example, one limit on the size of a typical HLA such as P1 is due to the limits on the maximum value of the pressure that the oil in first chamber P2 should reach when in use. The pressure of the oil in the first chamber P2 is dependant, among other things, on the diameter of the plunger assembly P4. Hence for a given engine load, there is an associated minimum diameter of the plunger assembly P4 (and hence outer body P3) required so that the pressure in the first chamber P2 does not exceed the specified maximum value. For a given load, therefore, typical HLAs such as P1 have a given minimum size.
It is desirable to provide an improved apparatus for hydraulic lash adjustment, preferably one with a reduced space burden as compared to conventional HLAs such as P1.